What is the ruling on praying behind an Imam who mispronounces Al-Fatiha in a way that alters its meaning, while intending to pray individually and only outwardly following the Imam, and are these prayers valid?

If the Imam does not recite Al-Fatihah correctly, with a grammatical error (laḥn) that alters the meaning, then the prayer behind him is not valid for a worshiper who can recite it correctly.

However, if a worshiper prays behind an Imam who makes mistakes in Al-Fatihah, without the intention of following him in prayer, but merely intending to follow his actions, scholars have differed regarding the validity of his prayer. The preponderant opinion is that his prayer is valid, because he has performed the pillars of prayer correctly.

It is recommended for anyone who knows that the Imam does not recite Al-Fatihah correctly to seek another mosque. If unable to do so, he should pray in congregation with his household or individually. He should also gently advise the Imam to correct his recitation.